The present disclosure relates generally to inverters connected in parallel, more specifically to uninterruptible power supplies (UPSs), and particularly to control systems for economic control thereof.
UPSs are employed in a variety of applications where a constant source of power is desired at a load. Double conversion UPSs are often the preferred choice for critical load protection. Double conversion UPSs offer the maximal protection level as the load is always fed by the inverter. Particularly, double-conversion transformer-based UPSs offer load isolation in addition to complete power conditioning. While providing optimum protection, double conversion transformer-based topology may not offer the highest efficiency. Additionally, the system load in some installations may be significantly far from optimal UPS efficiency (particularly in redundant 2N, 2(N+1) installations).
Methods for providing efficiency improvement in low-load conditions by switching “off” some UPSs (or section of the UPSs) in parallel UPS installations have been known. However, significant load steps exceeding available capacity will force the system to switch the critical load to bypass while additional units are brought on-line. This potentially exposes the load to power quality events occurring on the incoming utility. This issue particularly affects transformer-based UPSs, where the transition from off-line to on-line operation requires the transformer to be magnetized. Therefore, such known methods are typically applied only when the critical load is either stable or has predictable variation.
For parallel UPS systems, switching off one or more sections of the UPSs can reduce UPS losses. One particular solution is to switch off the inverter section of one or more units in low-load conditions and re-starting these inverters as the load increases. However, in transformer-based UPSs the inverter start-up is somewhat lengthy, as a soft-start is required when energizing the inverter output transformer.
Accordingly, there is a need for a system that allows the use of a double conversion UPS that overcomes the aforementioned drawbacks.